Personally, I'd set a limit that I would spend for the games I want to giveaway. I don't know how to explain this exactly. For example, if I would take $30 as my limit, then I would buy one copy of a $30 game, two copies of a $15 game, 6 copies of a $5 game (and so on), and make giveaways according to that. Every giveaway would have the same total value.
That way, I could giveaway copies of major titles while promoting good games of smaller companies.
I'd give priority to the games that I enjoyed, then to the public demand, then to interesting titles that I'd like to play, then to metacritic scores.

Wishlist is there for a reason ;)

1000 copies of Fortix :P

Games that haven't been put up yet!

check "our wishlist" and choose

personally I would choose some good games, but most of the people doesn't already have. (Ys: The Oath in Felghana, Recettear, Front Mission Evolved, World in Conflict, Dragon Age Ultimate Edition etc.)

or simply loads of Capcom games (except DMC3)

Garry's Mod
Terraria
Call Of Duty: MW3
Payday: The Heist
Killing Floor
Orange Box (Half-Life games?)
Rage
F.E.A.R. 3
Anno 2070
Age of Empires® III: Complete Collection
Plants vs Zombies GOTY Edition

rockstar complete pack is nice :)

Ys: The Oath in Felghana

Puzzle Quest 1 and 2

Shoot Many Robots

Retro City Rampage [when its released http://store.steampowered.com/app/204630/]

Legend of Grimlock

Other random Indie Bundles/Games

Everybody loves indie bundle.

Dear Esther.

Legend of Grimrock.

Ys: The Oath in Felghana.

Shoot many robots.

Alice: Madness returns.

Crysis 2.

Trine 2.

CaveStory+.

LA Noire.

Ridge Racer: Unbounded.

I love you. No-homo.

Tis not on Steam (yet), and god knows when will be, but Overgrowth FTW.

Maybe you could wait for the winter sale? And Gift acording of what is good and has a huge discount?
EDIT: Summer sale, my bad, im in the south :P

you should talk to Googly, and ask him how he did that gigantic maze he made, that was incredibly fun. Or do something else unique and exhausting, because it's more fun to torture people than to give them gifts, but doing BOTH is the epitome of fun. I don't really have suggestions on the specific games, except to say that odd indie games that people don't normally buy (The misadventures of P.B. Winterbottom, Sequence, etc...) are what I consider the most interesting giveaways.

DLCs!
Because generally speaking, I can't bring myself to buy them and so I usually like DLC giveaways more than game giveaways (partially because there are usually fewer entries).

And/or game soundtracks that are on Steam because those are pretty rare here.

wait for steams summer sale and get a few things from the daily sales?

Every deal of the day, mid-week and weekend deal until you run out of money.

21 copies of skyrim!

(jk, but how about some copies of Trine 2 and Mirrors Edge?)

Ninja Reflex, The Misadventures of P.B. Winterbottom, Binding of Isaac... these and other cheap good games would surely be very welcome.

How about backing The Banner Saga for a nice amount and using the rewards as giveaways if legal ?

Trine 2 giveaways always have more than 5k entries. It's multiplayer, wanted, cheap and indie. I'd buy at least 50 and I'd make a steam group for all the winners so they could contact each other to play together and help them become friends for the rest of their lives.

I like bananas.

You can give me Chantelise and Fortune Summoners!

Okay, better suggestion:

100 copies of Terraria.

Or, 50 copies of Terraria. Use the other $500 to fund a

STEAMGIFTS TERRARIA SERVER!!

Hypothetically speaking, you should donate all of that money to charity.